Recently developed powered-lift systems have shown significant potential to permit future aircraft, utilizing such systems, to operate from much smaller, isolated airfields, or conversely, to operate from existing fields with greatly increased payloads. Powered-lift systems commonly employ the jet exhaust from the engines of the aircraft in combination with a flap system on the wing to divert the engine exhaust downwards and thus increase lift.
One method of achieving this result is by blowing the jet exhaust over the upper surface of the wing, and utilizing downwardly extending trailing edge flaps to divert the exhaust, employing the well known Coanda effect. Such a powered-lift arrangement is commonly known as Upper Surface Blowing (USB), one such aircraft being described in U.S. Pat. No. 3,977,630 to Lewis et al. In USB, the engines are typically mounted on top of the wing with the engine nacelles extending forward of the leading edge of the lifting wing.
Over the Wing (OTW) blowing, another method for facilitating powered-lift, locates the engines, using pylons or the like, sufficiently above the wing so that the exhaust stream and wing surface do not interact. During powered-lift operation, a deflection hood, mounted on the upper perimeter of the engine exit, is rotated downward to deflect the thrust onto the aerodynamic high-lift system. As with USB, OTW locates the engine above the wing, with the engine nacelles being positioned forward of the leading edge of the wing.
Yet another powered-lift configuration is the Externally Blown Flap (EBF) wherein, in one embodiment, a portion of the efflux from the fan section of the jet engine passes under the wing. A trailing edge flap, often slotted, is utilized to direct the jet flow downwards during powered-lift operation. One such aircraft is disclosed by U.S. Pat. No. 3,614,028 to Klechner. In the EBF configuration, the engines are typically attached to the undersurface of the wing with the engine nacelles extending forward of the leading edge of the lifting wing.
In each of the powered-lift arrangements discussed above, a characteristic flowfield is generated. This flowfield is characterized by a region of upwash, at a relatively large angle, ahead of the lifting wing.
The high-lift coefficients provided by these advanced powered-lift systems are capable of producing flight speeds so low that conventional aerodynamic control surfaces cannot guide and stabilize the aircraft. Further, these powered-lift systems inherently cause nose-down pitching moments during operation. To trim these moments, the conventional approach has been an aft-mounted horizontal stabilizer which is loaded with a down force, i.e. negative lift, which produces nose-up pitch but reduces total aircraft lift available. However, at the slow flight speeds now obtainable, the dynamic pressure producing the tail down-load to trim has also been reduced while the wing pitching moment has become much larger. Thus, the trim problem has increased, for the ability of the tail to solve it has decreased. The required tail area to trim these moments has grown to the point of being unacceptable from a cruise drag, complexity, and weight standpoint.